4 added significant advantages over previous versions, like read-only and read/write folder mounting capabilities, splitting to multiple image files (SWM), a WIM filter driver and the latest compression algorithms. It was first introduced in Milestone 4 into the Longhorn project and used in later builds of Longhorn. It allowed Microsoft OEM partners to experiment with the imaging technology and was developed in parallel with Longhorn alpha prototypes. The first distributed prototype of ImageX was built.
Starting with Windows Vista, Windows Setup uses the WAIK API to install Windows. Along with the underlying Windows Imaging Interface library (WIMGAPI), It is distributed as part of the free Windows Automated Installation Kit (WAIK/OPK). ImageX is the command-line tool used to create, edit and deploy Windows disk images in the Windows Imaging Format.
Both solid compression and LZMS are introduced more recently, in WIMGAPI Windows 8 and DISM Windows 8.1. There is also support for solid compression. The former two use Huffman encoding, while the latter uses adaptive Huffman encoding with range coding. WIM supports three families of LZ77-based compression algorithms in ascending ratio and descending speed: XPRESS, LZX, and LZMS. Other setup files are held in the INSTALL.WIM. In this case, BOOT.WIM contains a bootable version of Windows PE from which the installation is performed. Windows Setup DVD in Windows Vista and later use such WIM files. Windows boot loader supports booting Windows from within a WIM file. WIM images can be made bootable (WIMBoot). A WIM can also be split (spanned) into multiple parts, which have the. Due to the use of single-instance storage, the more each successive disk image has in common with previous images added to the WIM file, the less new data will be added. WIM files can contain multiple disk images, which are referenced either by their numerical index or by their unique name. The cost of reading or writing many thousands of individual files on the local disk is negated by hardware and software-based disk caching as well as sequential reading and writing of the data. Since the files are stored inside a single WIM file, the overhead of opening and closing many individual files is reduced. The primary advantages of being file-based is hardware independence and single-instance storage of a file referenced multiple times in the filesystem tree. However, unlike sector-based formats (such as ISO or VHD), WIM is file-based: The fundamental unit of information in a WIM is a file. Like other disk image formats, a WIM file contains a set of files and associated filesystem metadata.